Evolution of Mitochondrial Introns in Plants and Photosynthetic Microbes

Introns within the mitochondrial genes of plants and algae are categorized as group I or group II based on distinctive structural properties that are important for splicing. These families of intron ribozymes are mobile genetic elements that can invade genomes through horizontal transfer and spread copies of themselves during evolution. Fully competent group I and group II introns encode their own machinery for splicing and mobility, but those found in present-day plant/algal mitochondria lack certain features and virtually none have been shown to be self-splicing in vitro. Accessory proteins are required for splicing in vivo and such machinery can include nuclear-encoded homologues of intronic RNA maturases as well as other RNA-binding proteins, as determined primarily from studies of Arabidopsis mutants. Several mitochondrial introns in plants/algae have been disrupted by DNA rearrangements during evolution and the independently-transcribed pieces are expressed through trans-splicing. The interplay between RNA editing and splicing can also confer an additional layer of complexity and coevolutionary constraint. This chapter highlights aspects of the evolutionary history of mitochondrial introns in plants and algae, the recruitment of host machinery for splicing, and the relationships between splicing and other expression events in the mitochondrion.